Yikes! What is that! "Scalars leaked: 1". Well, yes, that's one of the remaining problems/features/bugs of the Perl ithreads implementation. This particularly seems to happen when you start threads at compile time. From practical experience, I must say it seems to be pretty harmless.

The leaked scalar is the result of returning a closure from a thread. In many cases it is, as mentioned, harmless. However, in some implementations of Perl, returning a closure from a thread may cause the interpreter to crash (i.e., dump core). Caveat emptor.